This invention relates to a system for permitting multiple microprocessors to share the same video display, and, more particularly, for sharing the same display in such a way that video performance is not compromised.
Most conventional personal computer systems have a single microprocessor. In some circumstances, however, it may be advantageous to have one or more additional microprocessors additionally integrated into the system. For example, if the primary system processor is a 60x (e.g., 601, 603, 604) microprocessor, it may be desirable also to include a DOS-based microprocessor such as an Intel 486 or Pentium microprocessor as an auxiliary processor to enable the system to run DOS programs.
A conventional system for achieving this end is shown in FIG. 1. As shown, the system 10 of FIG. 1 includes a primary microprocessor 20 and an auxiliary microprocessor 30. The primary microprocessor 20 may be on a motherboard such as motherboard 25 and the auxiliary processor 30 on an expansion card 40, although other arrangements are possible. Either or both processors may be 60x series processors, x86 type processors, or any other suitable type of microprocessor.
It is advantageous in such a system for the microprocessors to share system resources such as power supplies, I/O devices such the keyboard and disk drives, and output devices such as a monitor and printer. In the system 10 shown the microprocessors share a monitor 50. To do this, each microprocessor generates its own set of digital video signals which are conveyed to a respective digital to analog converter (DAC), that is, a DAC1 60 for the primary microprocessor 20 and a DAC2 70 for the auxiliary microprocessor 30. The respective analog video outputs of the DAC1 60 and the DAC2 70 are connected to respective video out connectors 65 and 75, and then summed at a video summing junction B. The interconnection of the video out connectors 65 and 75 to the summing junction B, and the connection of the summing junction B to the monitor 50, is accomplished by an external wiring harness 90.
Conventionally, a DAC such as either DAC1 60 and DAC2 70 which converts digital video into analog video is placed close to the external video out connector 80. This has the disadvantage that conveying the digital video signal from the DAC to the connector across the PCB can cause electromagnetic interference (EMI). It also has the disadvantage that it necessitates running a parallel digital signal across the board, which can require 32 or even 64 traces.
In a system 10 such as that shown, it is also possible to have significant signal reflection in the lines carrying the analog video signal. In effect, each DAC launches a forward-going wave toward the connectors 65 and 75 and the monitor 50. The wave encounters an impedance discontinuity when it reaches these elements. At the discontinuity, part of the wave's energy will be transmitted, but another part will be reflected. In a line which is assumed to be lossless, the magnitude of the reflected signal or wave is determined by the magnitude of the incident signal multiplied by a reflection coefficient which depends in turn on the magnitude of the difference between the line impedance and the load impedance. Ideally, if the line impedance equals the load impedance, then the impedances are matched, the reflection coefficient is zero, and there is no reflected signal. In practice, however, there is an intrinsic mismatch, so that reflections are created.
These reflections or return waves if not controlled or eliminated will distort forward-going signals in the line. For example, the theoretical ideal primary color waveforms for a linescan of a standard color-bar chart are shown in FIG. 2. These waveforms are taken from J. Fisher et al., "Waveforms and Spectra of Composite Video Signals", Television Engineering Handbook, p. 5.24 (McGraw-Hill Book Co. 1986). These waveforms are for the line scan of a chart having colors in the order of red, yellow, green, cyan, gray, magenta, blue, white, and black. The amplitude of the signal in each channel (R'.sub.R, E'.sub.G, and E'.sub.B) of the composite video signal is determined by the amount of the corresponding color in the scanned image, and will in turn determine the amount of that color in a video reproduction of that image. Reflections cause spurious "structure" (deviations from true signal levels) in each channel in the composite video signal. This structure may take the form of a stepwise increase or decrease in a signal ("shelving") which should be increasing or decreasing nearly instantaneously. The signal at a given point is in effect the superposition of the forward-going wave and the reflected return signal. This superposition obscures the data in the forward-going signal, thus distorting the color in the video image.